The process efficiency of a pulp mill is a complex function and it depends on for example the pulp type, the pulp properties, the process equipment and the conditions used. For the first process stages in a bleached kraft pulp mill, i.e. cooking, oxygen delignification, and the D0 stage, the challenge is to achieve optimum delignification and high selectivity i.e. obtaining low lignin content and high pulp yield and viscosity. A second challenge is to maximize the pulp production at the required final brightness and strength level.
There is a large impact of operating cooking and oxygen delignification stages properly. Operating too far away from desired working points may result in significant loss of productivity, though sometimes it is very difficult to determine directly and quantitatively, in terms of e.g. pulp yield and strength. From a process control point of view, disturbance factors will contribute to process variability and result in offsets from the target set-points. Such disturbance factors may be related to e.g. raw material, such as chip moisture, liquor strengths, wood composition etc. It may also relate to inaccurate or improper process sensors being used, for example consistency, flow velocity, pH etc. While not every single property and constituent can be measured in every process stage the inherent challenge is to identify the key process parameters, to measure and base process control solutions on those parameters, in order to achieve a high productivity.
The degraded and dissolved lignin residues will ultimately go to the recovery boiler. However some downstream “leakage” will naturally occur and that is the black liquor carryover. Furthermore, even though the majority of the delignification occurs in the cooking stage, still a quite significant Kappa number reduction takes place in subsequent stages and this will add more dissolved lignin to the process streams.
This black liquor carryover, referring both to organic and inorganic constituents, is well known concerning its composition, and there are sensor technologies used today that try to capture the carryover. However they do not measure specifically the dissolved lignin content which has been identified being the most critical parameter in pulp washing but rather the content of inorganic matter in the black liquor.
The lignin content in the process fluid can be determined by measuring the absorption of preferably ultra violet (UV) light. However, this implies that the fibres have been separated and can thus be applied on-line only on a relatively pure fluid flow, i.e., with no fibres. Some compensation for the fibre content (including fibre fragments and other light scattering particles) can be achieved by performing the measurement at two or more wavelengths, but even at relatively low fibre contents these fibres block almost all light when measuring for example the light transmittance whereby the determination of the fluid phase will be interfered or made impossible.
The patent document SE 464 836 describes that due to the low number of large particles, such as fibres, present in a given small measuring volume it will statistically be occasions when there will be no large particles in this volume, provided that the concentration is sufficiently low relatively to the volume. However, the number of small particles is high also in a small volume, such as 1 mm3, and statistically the concentration of small particles will remain essentially constant over time.
FIG. 1 shows an explanatory sketch for measurement of transmittance with a high frequency, with a small measuring volume containing a suspension with both large and small particles. The graph shows signal level as a function of time, wherein VCW is the signal level for pure water, VP is the top level of the signal and VDC is the average signal level. FPC corresponds to the concentration of small particles and LPC corresponds to the concentration of large particles.
When measuring the light attenuation, or inversely transmittance, when the medium flows through a given measuring volume, the signal level will be the highest when there are no large particles in the measuring volume, i.e., the top level, see illustration d) in FIG. 1, and correspond to the concentration of small particles, see illustrations a) and c) in FIG. 1. The average signal level is influenced by both large and small particles, see illustration b) in FIG. 1, which means that the concentration of large particles can be determined from the average signal level together with the top level, calculated in a suitable way. The method is graphically described in FIG. 1, which thus is general and applies for measurement of light attenuation, i.e., transmittance, although it in principle can be applied also when measuring reflectance.
By using a small measuring volume and a high time resolution, a differentiation between large and small particles is made possible. It is thus an advantage that the disproportionally large contribution of the large and small particles to the measuring signal can be compensated for and that the concentration of both large and small particles, and also the total particle concentration, can be obtained.
However, determining the properties of individual phases in a heterogeneous medium remains a problem.